1. Field of Invention
The present invention generally relates to overprint compositions for xerographic prints. The overprint compositions provide a number of advantages to xerographic prints, such as, for example, image permanence, thermal stability, lightfastness, and smear resistance. In addition, the overprint compositions reduce document offset.
2. Description of Related Art
In conventional xerography, electrostatic latent images are formed on a xerographic surface by uniformly charging a charge retentive surface, such as a photoreceptor. The charged area is then selectively dissipated in a pattern of activating radiation corresponding to the original image. The latent charge pattern remaining on the surface corresponds to the area not exposed by radiation. Next, the latent charge pattern is visualized by passing the photoreceptor past one or more developer housings comprising toner, which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate, such as paper, to which it is fixed by a suitable fusing technique, resulting in a xerographic print or toner-based print.
Although xerographic equipment is used worldwide, it possesses a significant disadvantage in that the energy consumption is quite high. Thus, equipment with lower power consumption has been designed. Toners that function in the lower power consumption equipment, known as “low-melt toners,” are designed to have low glass transition temperatures (Tg's) of about 55° C. to about 65° C. However, an image defect known as document offset (or “blocking”) can occur at temperatures as low as about 54° C. to as high as about 70° C. or more when the toner begins to flow. Thus, low-melt toners often have a significant document offset problem. Document offset properties of various toners are set forth in Table 1.
TABLE 1Comparison of Document Offset Propertiesof Various Low-Melt TonersTonerMachineTemperature*FC IIDC2060 & DC1262°C. (144° F.)FC IDC40 & Majestik ® (Xerox Corp.)61°C. (142° F.)5090DT18055.5°C. (132° F.)C6 & M4iGen3 ® (Xerox Corp.)55.5°C. (132° F.)*where Document Offset (DO) = 4.0 @ 10 g/cm2
At document offset-provoking temperatures, when combined with pressure, such as several reams of paper in an output tray of a printer, the toner sticks to the sheet above it, or, in the case of duplex printing, the toner on the sheet above it. This yields two sheets that have to be pulled apart. In the worse case scenario, the toner pulls off part of the image on or paper fibers from the sheet above it. Clearly, this results in a loss of quality of the toner-based print (also referred to as a toner-based image, xerographic print, or xerographic image).
Known methods of reducing document offset include adding wax to the toner and applying an overprint coating to the substrate. The overprint coating, often referred to as an overprint varnish or composition, is typically a liquid film coating that may be dried and/or cured. Curing may be accomplished through drying or heating or by applying ultraviolet light or low voltage electron beams to polymerize (crosslink) the components of the overcoat. However, known overprint coatings, such as those described in U.S. Pat. Nos. 4,070,262, 4,071,425, 4,072,592, 4,072,770, 4,133,909, 5,162,389, 5,800,884, 4,265,976, and 5,219,641, for example, fail to adequately protect xerographic prints and fail to reduce document offset.
In addition, known coating formulations fail to prevent the formation of hairline cracks on the print surface in response to thermal expansion of the toner, which creates an undesirable appearance. This is a particularly important issue for automobile manuals, book covers, etc., which require the prints therein to survive high temperatures for hours at a time, yet retain a neat appearance.
Moreover, known coating formulations fail to protect xerographic prints from bead-up and smears caused by overwriting on the print with liquid markers. The ability to neatly overwrite without beading and smearing is vital for numerous commercial applications, such as, for example, restaurant menus and calendars.
Accordingly, a need exists for a xerographic print protective composition that provides overprint coating properties including, but not limited to, thermal and light stability and smear resistance, particularly in commercial print applications. More specifically, a need exists for an overprint coating that has the ability to wet over silicone fuser oil (generally found on xerographic substrates), permit overwriting, reduce or prevent thermal cracking, reduce or prevent document offset, and protect an image from sun, heat, etc. The compositions and processes of the present invention, wherein a xerographic print is coated with a radiation curable overprint composition, satisfies this need.